The present invention relates to an optical alignment apparatus of the type that, for example, is used to provide alignment with a location having a physiological property to be measured, such as with respect to an eye. The present invention also relates to a method of providing optical alignment with a location having a physiological property to be measured, such as with respect to an eye.
Diabetes is a major and rapidly growing problem with over 230 million people suffering from the disorder worldwide. In addition, studies have shown that the incidence of juvenile-onset, insulin-dependent diabetes has doubled over the last 15 years. There has also been a five fold increase in the number of children under the age of 5 suffering from diabetes in just 20 years.
The symptoms associated with diabetes can be severe. If the blood glucose level is not suitably controlled by the patient, the physical damage which may be caused includes blindness, heart disease and gangrene. As such, the mortality rate for people with diabetes is significantly higher than the rate for the average person.
A person's blood glucose concentration varies over a relatively short timescale, due to a number of factors, such as the length of time since the patient's last meal, the type of food ingested, the amount of exercise taken, and whether or not the patient is otherwise ill. As a result, people with diabetes usually need to test their glucose levels many times a day, in order to monitor and control their condition. The actual testing regime varies between patients and is individually prescribed by the doctor or diabetes educator of the patient.
The primary method used for testing blood glucose concentration involves the taking of a blood sample, which is then analysed. In this test, a patient's finger or arm is pricked with a small needle and the resulting drop of blood is placed on a test strip, for analysis in a hand-held meter. If the glucose concentration reading is above an acceptable level, insulin must be injected to bring the glucose concentration back within an acceptable range.
Due to the frequency of testing required to monitor the blood glucose concentration, the patient is normally expected to perform the tests throughout the day, drawing and analysing the blood sample himself. There are a number of problems experienced by patients with the above procedure. Firstly, the technique is invasive and therefore carries the risk of infection. Secondly, continual pricking of the fingers causes hard skin. Thirdly, the process is clearly not pain-free. Finally, there is a large, ongoing consumables cost associated with this method. As a result of these and other problems, certain sectors of the diabetic population do not test themselves as often as required. This is particularly the case for the elderly, who tend to lack the fine motor skills required; teenagers, who tend to find the whole procedure socially embarrassing; and children, who tend not to accept the discomfort associated with the process.
A number of non-invasive blood glucose concentration measuring techniques have been proposed to overcome these problems. In general these have been designed to work by making a measurement through the skin but the variability in the skin's characteristics have led to inaccurate results.
More recently the eye has been proposed as a better measurement location. Possible techniques for measuring glucose in the eye include spectroscopy on the conjunctiva (e.g. U.S. Pat. No. 6,975,892), psychophysical measurements on the fundus (e.g. U.S. Pat. No. 6,895,264), a contact lens or other implantable device that absorbs glucose (e.g. U.S. Pat. No. 6,980,842 or US 2006/0166350) or a measurement of the ocular refractive correction (e.g. U.S. Pat. No. 6,442,410).
One particular approach which has been suggested involves measuring the glucose concentration of the aqueous humour in the anterior chamber of the eye, since, while varying between individuals, there is a close correlation between this concentration and the blood glucose concentration. Measurement of the glucose concentration of the aqueous humour may be achieved by various means; for example, by polarimetry (e.g., U.S. Pat. No. 5,896,198); by Raman techniques (e.g., WO A 00/02479); by fluorescence photometry (e.g., WO 2005/120334); by spectrometry (e.g., U.S. Pat. No. 5,969,815); by fluorescence spectroscopy (e.g., WO 02/087429) or by reflectometry (e.g., U.S. Pat. No. 6,236,089).
A desirable alternative approach to measuring the glucose concentration in the aqueous humour involves measuring the refractive index of the aqueous humour, since there is a strong correlation between the refractive index and the glucose concentration. In this respect, U.S. Pat. No. 3,963,019, U.S. Pat. No. 6,152,875, WO 03/025562, WO 05/044099 and WO 05/058152 describe various techniques associated with measurement of the refractive index of the aqueous humour.
In addition there are many other measurements that require an instrument to be aligned to the eye of a patient or operator. In one example, it is necessary to measure the thickness or shape of the cornea in order to make Laser-Assisted in Situ Keratomileusis (LASIK) surgery safer (for example as described in U.S. Pat. No. 6,585,723 and US 2004/0080759).
In all of the above cases, the measurement fidelity is compromised by variations in alignment between the meter and the patient's eye. In addition, for a successful personal use meter, it is important that the patient is able to use the meter by themselves, and align to the meter by themselves, without any clinician involvement.
Furthermore, at times it is not always possible to achieve good alignment to the eye when measuring glucose levels and other parameters of the eye non-invasively.